The present invention relates to mobile communication equipment, and particularly to a direct-conversion transmitting circuit suitable for large scale integration and to an integrated transmitting/receiving circuit using the same.
According to rapid spread of the mobile communication equipment, requests for miniaturization and lower cost thereof have increased. Because of this, it is expected to apply a voltage control type oscillator (VCO), or an integrated circuit whose filter number is reduced and whose integration is enhanced. What is given as one conventional example of transmitting equipment is “RF Circuits Technique of Dual-Band Transceiver IC for GSM and DCS1800 applications” published in pages 278 to 281 of manuscripts for IEEE 25th European Solid-State Circuits Conference on 1999 by K. Takikawa et al.
As an important item on a transmitting circuit design, reduction of noise leakage into receiving frequency band has been given. FIG. 2 shows a relationship between transmission power defined by specifications for European cellular phones (GSM) and noise generated in receiving band. As indicated by an allowed output spur level 202 in a GSM receiving band, it is required that noise in receiving a band 204 at slightly 20 MHz distant from an upper limit of a transmission band 203 is suppressed up to −79 dBm/100 kHz (−129 dBm/Hz) or less relative to a maximum output power of 33 dBm in a GSM output signal 201. That is, a difference between a transmission signal and a noise level is required to be −112 dBc or more. If a band pass filter or the like is applied to an output portion of a power amplifier, the above-mentioned specification can be achieved. However, decrease in efficiency thereof is generated due to influence on losses of the filter. Thus, an offset PLL is applied as a constitution using no filter.
FIG. 18 is a circuit constitution diagram showing a transmitter to which the offset PLL is applied. The transmitter is composed of an IF signal generating section 1815 and a PLL section 1814. First, an operation of the IF section will be described here. I and Q signals each having a band of 200 kHz are input. These input signals are mixed in intermediate frequency (IF) local signals 1812 and 1813 each having a phase difference of 90° and mixers 1808 and 1809, respectively. In this case, the local signals 1812 and 1813 are obtained by phase-shifting an output of the VCO 1806 by a 90° phase shifter 1807. By adding outputs of the mixers 1808 and 1809, the input signals are converted into a GMSK (Gaussian Minimum Shift Keying) modulation signal of an IF frequency (270 MHz), respectively. The GMSK modulation signal is a modulation signal adopted in the GSM system, and has signal information only in a phase, and is constant in amplitude. In order to provide a sufficient amplitude to a phase comparator 1802 located at a downstream side of the circuit, the IF signal is amplified at an amplifier 1810. After harmonics generated by the mixers 1808 and 1809 and the amplifier 1810 are removed by a low-pass filter 1811, the IF signal is input to the phase comparator 1802 of the a PLL section 1814.
The PLL section 1814 is characterized by including a mixer 1801, and converts a frequency of an output signal of the VCO 1800 operated by an RF frequency, into an IF frequency fIF (270 MHz), by means of a mixer 1801, and outputs amounts of error between the IF signal and the output signal of the VCO 1800 by means of the phase comparator 1802. A frequency of the output error signal is lowered up to a baseband signal band that is the same as the I and Q input signals. High frequency noise of the error signal is suppressed by the low-pass filter 1803. A cutoff frequency of a PLL closed loop of a filter, the PLL closed loop which is denoted by reference numeral 1816, is about 1.6 MHz in a signal band of 200 kHz, and a noise of 20 MHz is greatly suppressed. Because of this, the noise generated in band which is a 20 MHz distant from an output signal of the VCO 1800 is greatly suppressed. Therefore, even if an output of the VCO 1800 is directly connected to a power amplifier PA, it is possible to suppress noises of receiving band up to −79 dBm/100 kHz (−129 dBm/Hz) or less without newly connecting a filter to an RF signal.
In a transmitter using the offset PLL, although a portion 1817 enclosed in solid line shown in FIG. 18 is integrated, the VCO 1800 is an external part because low noise characteristics are required. However, if the offset PLL is used, an external filter for high frequency is not required. Therefore, it is possible to be widely applied as a transmitter with high efficiency.